Heretofore, as one of fuel injection systems for a diesel engine, a common rail fuel injection system (i.e., accumulator-type fuel injection system) has been developed. In the common rail fuel injection system, high-pressure fuel pressurized by a fuel pump is accumulated in a common rail (i.e., accumulator), and an electromagnetic valve disposed in a fuel injection nozzle is so controlled as to open and close a needle valve disposed in the fuel injection nozzle, which makes it possible to achieve the control in fuel injection.
For example, Japanese Patent Laid-Open No. Hei 6-93936 discloses an accumulator-type fuel injection system, which is shown in FIG. 12. Now, with reference to FIG. 12, an example of the common rail fuel injection system will be described. In the common rail fuel injection system, there are provided: a high-pressure common rail (hereinafter referred to as the high-pressure accumulator or high-pressure fuel accumulator) 103 for storing high-pressure fuel therein; and, a low-pressure common rail (hereinafter referred to as the low-pressure accumulator or low-pressure fuel accumulator) 104 for storing low-pressure fuel therein.
Supplied to the high-pressure accumulator 103 of these accumulators is the fuel having been pressurized to a predetermined value by a high-pressure fuel pump 101, while supplied to the low-pressure accumulator 104 is the fuel having been pressurized by the high-pressure pump 101 and having passed through a pressure-regulating valve (i.e., regulator) 118 to decrease in pressure.
Further, provided in a downstream side of the low-pressure accumulator 104 is a three-way electromagnetic valve (i.e., second three-way electromagnetic valve ) 107. Connected with this second three-way electromagnetic valve 107 are: a fuel passage 110b from the above-mentioned low-pressure accumulator 104; a fuel passage 110a from the high-pressure accumulator 103; and, a communication passage 110d communicating with a fuel chamber 112 of a fuel injection nozzle (i.e., injector) 109.
This second three-way electromagnetic valve 107 is so constructed as to be subjected to a switching control for selectively supplying the high-pressure fuel from the high-pressure accumulator 103 and the low-pressure fuel from the low-pressure accumulator 104 to the fuel chamber 112 of the injector 109. Incidentally, in this case, when the second three-way electromagnetic valve 107 is held closed (i.e., held in its OFF position), the fuel passages 110b and 110d are communicated with each other to permit the low-pressure fuel to be supplied from the low-pressure accumulator 104 to the fuel chamber 112. On the other hand, when the second three-way electromagnetic valve 107 is held open (i.e., held in its ON position), the fuel passages 110a and 110d are communicated with each other to permit the high-pressure fuel to be supplied to the fuel chamber 112, which stops the supply of the low-pressure fuel.
Further, as shown in FIG. 12, in this injector 109, there is provided a three-way electromagnetic valve (i.e., first three-way electromagnetic valve) 105 for controlling the fuel injection. As shown in the drawing, connected with this first three-way electromagnetic valve 105 are: a fuel passage 110a from the high-pressure accumulator 103; a fuel return passage 110c; and, a fuel passage 110e communicating with a control chamber 111 of the injector 109.
And, the first three-way electromagnetic valve 105 is so constructed as to be subjected to a switching control for: supplying the high-pressure fuel from the high-pressure accumulator 103 to the control chamber 111 of the injector 109; and, discharging the high-pressure fuel, which has been supplied to this control chamber 111, into a fuel tank 117.
In this case, this first three-way electromagnetic valve 105 is adapted to: connect the fuel passage 110a from the high-pressure accumulator 103 with the control chamber 111 when a control signal from a controller 108 described later is OFF; and, connect the control chamber 111 with the fuel return passage 110c when the control signal from the controller 108 is ON.
On the other hand, provided in this control chamber 111 is a hydraulic piston 114 which is in abutting engagement with a needle valve 113 of the injector 109. In operation, this hydraulic piston 114 is controlled by the high-pressure fuel which is supplied to the control chamber 111 through the above-mentioned fuel passage 110e. When the hydraulic piston 114 moves down to urge the needle valve 113 downward, an injection hole of the nozzle's front end is closed by the needle valve 113 to prevent the fuel from being injected.
Further, a check valve 106 and an orifice (i.e., throttle) 115 both communicating with the fuel passage 110e extending to this control chamber 111 are connected in parallel with each other. When the first three-way electromagnetic valve 105 is held in its OFF position, the high-pressure fuel from the high-pressure accumulator 103 is promptly supplied into the control chamber 111 mainly through the check valve 106. On the other hand, when the first three-way electromagnetic valve 105 is held open, the high-pressure fuel within the control chamber 111 is drained through the orifice 115 at a relatively slow rate.
Further, in this common rail fuel injection system, there is provided the controller (i.e., ECU) 108. The controller 108 is adapted to receive engine's rotation speed information Ne, fuel pressue information P.sub.HP in the high-pressure accumulator 103, fuel pressure information P.sub.LP in the low-pressure accumulator 104, accelerator's opening angle information Acc and like information. In the controller 108, based on these pieces of information Ne, P.sub.HP, P.sub.LP, Acc, control signals for controlling the electromagnetic valves 105, 107 and the regulator 118 in operation are prepared and issued to these valves 105, 107 and the regulator 118.
In such a common rail fuel injection system, when the fuel injection is not performed, the controller 108 controls both the first three-way electromagntic valve 105 and the second three-way electromagnetic valve 107 so as to held them in their OFF positions. Due to this, the low-pressure fuel from the low-pressure accumulator 104 is supplied to the fuel chamber 112 of the injector 109, and the fuel under high pressure is supplied to the control chamber 111 of the injector 109 through the check valve 106.
In this case, since the high-pressure fuel acts on the hydraulic piston 114 of the control chamber 111, the hydraulic piston 114 is forced to move down. Consequently, the needle valve 113 abutting against this hydraulic piston 114 is also forced to move down to close the injection hole of the injector 109, which prevents the fuel from being injected.
When the fuel is injected, at first, only the first three-way electromagnetic valve 105 is so switched as to be held in its ON position. Due to this, the high-pressure fuel in the fuel chamber 111 is gradually drained through the orifice 115, and the needle valve 113 is opened under the influence of the fuel pressure in the fuel chamber 112 to start the fuel injection.
After that, while holding the first three-way electromagnetic valve 105 in its ON position, an operation for switching the second three-way electromagnetic valve 107 so as to hold it in its ON position is performed. Due to this, the high-pressure fuel is supplied to the fuel chamber 112 of the injector 109, so that the fuel injection of such high-pressure fuel is performed.
When the fuel injection comes to the finish, both the first three-way electromagnetic valve 105 and the second three-way electromagnetic valve 107 are held in their OFF positions, which permits the high-pressure fuel to be supplied into the control chamber 111 through the check valve 106, so that the needle valve 113 is promptly moved down to stop the fuel injection. Further, during such downward stroke of the needle valve 113, the low-pressure fuel is supplied into the fuel chamber 112 for preparation of a subsequent one of fuel injection cycles.
However, in such conventional type of common rail fuel injection system, since expensive three-way electromagnetic valves the number of which is two are used, there are problems of increasing the cost and of making it difficult to reduce in size the fuel injection system itself.
In view of these problems, the present invention was made. Consequently, it is an object of the present invention to provide a fuel injection system which is capable of minimizing the cost and also capable of being reduced in its size.